Flash memory commonly uses Fowler-Nordheim tunneling for programming memory cells. The tunneling current is dependent on the voltage gap between a control gate and a channel region. When programming memory cells, a program voltage is applied to a word line containing memory cells being programmed and other cells which are to be inhibited from programming. Program disturb occurs when programming of memory cells causes an inhibited memory cell on the same word line to become inadvertently programmed.
Program disturb is expected to be a key limiter of NAND memory cell scaling. As the size of memory is scaled down, the natural variation in the cell to cell programming behavior increases. As a result, it becomes harder and harder to keep a fast programming cell which is in an erased state from disturbing up while ensuring that a slow programming cell can program to a target threshold voltage Vt.
During programming, unselected word lines are supplied a voltage to inhibit the memory cells on the unselected word line so that the memory cells are not programmed. The voltage goes into the channel region, which is boosted from coupling with the inhibited word lines and improves program disturb. Voltage, however, leaks out of the channel region.